It has been found that the most efficient way to convert an alternating current (AC) signal of a first frequency to an alternating current signal of a second frequency or to invert a direct current (DC) signal to an alternating current signal is to use pulse duration modulation techniques. Reference can be made to the National Technical Information Service publication of the U.S. Department of Commerce, No. AD-A036 027 published January 1977 entitled "Trade-Off Investigations and Detailed Design of Power Inverters" by W. G. Lawrence.
In most inverter or converter systems, the input signal is applied to an input conditioner which is used to reduce the electromagnetic interference, perform voltage transformation, and in some cases provide isolation, regulation and protection for the remainder of the circuitry. The output of the input conditioner, which should be a direct current signal, is conducted to what is commonly referred to as the inverter power stage which performs the basic functions of the inverter by changing the direct current signal into an alternating current signal and provides regulation when not accomplished in the input conditioner. The output of the inverter power stage is passed to the output conditioner which, in general, is selectable so as to provide a selection of voltages. In addition, the output conditioner includes output filtering and may include input/output isolation if it is not provided elsewhere in the converter.
Voltage selection and isolation can both be accomplished easily by the use of an output transformer. The transformer secondaries can be rewired in series or parallel to provide the desired output voltages. There are transformerless output conditioners also known in the art and the selection of either the transformer type output conditioner or the transformerless type output conditioner is based upon the design application of the circuitry. However, in cases of a low frequency output voltage, a transformer if used, would be extremely bulky and in this case, the transformerless type output conditioner might be preferable. However, for AC to AC converter applications which require isolation, the transformer is part of the input conditioner or the output conditioner in accordance with the higher frequency.
The inverter power stage, because it controls the basic functions, is what determines the efficiency, accuracy and response time of the converter system. It has been found that the most advantageous type of inverter uses the technique of generating a high frequency rectangular waveform which is pulse duration modulated to synthesize the output sinewave while simultaneously controlling the amplitude and frequency of the output. The output filtering requirements are greatly reduced in that the filter need only remove the high frequency switching frequency from the output signals. Filter design is, therefore, relatively independent of the output frequency and the same filter may be used for the 60 hertz and 400 hertz outputs or for any other reasonable frequency design. An additional advantage is that the filter impedance is lower at the output frequency thus reducing the phase shift across the filter and minimizing the problems of parallel operations and operations with unbalanced loads.
In the prior art inverters as well as the inverter disclosed herein, the inverter power stage acts as a power amplifier for signals developed at lower levels by a signal generator and controller. Each inverter power stage requires the generation of single phase and/or three phase outputs of closely controlled frequency and amplitude. Alternative sine-wave sources include sine-wave oscillators, square-wave generators with filters, quasi-square-wave generators with filters and digital-to-analog converters.
It is a generally held opinion that the prior art sine-wave oscillator has two distinct disadvantages; two sets of frequency determining elements are required for the two outputs and for three-phase generation, the three oscillators must be kept precisely 120.degree. apart in their phasing.
The square-wave generator source is simple to produce but filtering becomes a problem along with the need to maintain proper phasing with changes in component values. While the quasi-square-wave type signal generator reduces the filter demands, a three level logic signal is required which is not readily available from standard logic elements.
It is generally known in the prior art as can be referred to in the above reference publication that digital-to-analog converters are the most efficient signal generators. In this publication, it is stated that the digital-to-analog converter, while seeming more complicated at first glance, has distinct advantages over the other signal generator circuits. Each of the three separate phases may be derived from a common counter and control network driven by a single crystal controlled oscillator. The number of steps provided in the converter for synthesizing the sine-wave may be chosen to provide low distortion and minimum filtering and phase shift. By suitable selection of the oscillator frequency a single crystal oscillator can be used to provide 50, 60 or 400 hertz outputs just by changing the countdown between the oscillator and the digital-to-analog converter. The crystal controlled oscillator feeds a frequency controlled signal to the divider through controlled circuitry. This controlled circuitry may include synchronizing circuitry to synchronize interconnected inverters and/or synchronize the signal produced to a possible pulse duration modulator. In response to an input control signal, the counter decoder can provide outputs for the desired output frequency. DC outputs of the counter decoder are fed to the digital-to-analog converter to control the instantaneous relative amplitudes of synthesized sine-waves, and other outputs are fed to the polarity control circuits to control the polarities of the signals.
The main disadvantage of the above-described techniques and those described in the above-referenced publication, is that the AC output from the output conditioner is rectified and filtered to form a DC analog signal. Although the preferred embodiment of the prior art techniques previously described permit low output distortion using minimum passive filtering for steady state resistive loads, the closed loop frequency response in all of the above-described techniques is relatively low. This is because the limiting item in the loop is the low frequency filter associated with forming the DC analog of the AC output voltage rather than the AC output filter itself. Thus, even the D to A converter technique, as described, will generate high voltage transients in response to step load changes and, perhaps more significantly, will produce a distorted output voltage in the face of reactive or nonlinear loads. What is needed is a control loop which eliminates the necessity of forming a DC analog of the AC output voltage, thus permitting a frequency response fast enough to cause the various pulse widths of the pulse width modulated inverter to be individually modulated in accordance with the instantaneous load demands.